The asymmetric hydrogenation of carbon-nitrogen double bonds in imines is known. Rhodium, iridium or titanium catalysts (J. Mol. Catal. 1990, 62, 243; J. Am. Chem. Soc., 1994, 116, 8952 and J. Am. Chem. Soc., 1990, 112, 9400) have hitherto been used. The imines have always been used as the free base. Ruthenium catalysts are known for specially functionalized substrates (Tetrahedron Letters 1990, 31, 4117).
The hydrogenation of the compound of formula II (formula II described below) as the free base is problematical, since it is unstable in this form and disproportionates to the corresponding 1-(4-methoxy-benzyl)-5,6,7,8-tetrahydro-isoquinoline (tetrabase) and also to rac-1-(4-methoxy-benzyl)-1,2,3,4,5,6,7,8-octahydro-isoquinoline (rac-octabase). Moreover, the known processes such as for example the production of dextromethorphan have the disadvantage of racemate resolution, since the known hydrogenation of a compound of formula II (formula II described below) in the presence of a heterogeneous catalyst leads to a racemate, which must be resolved, since only the (S)-enantiomer of formula I (formula I described below) is used. Furthermore, the undesired (R)-enantiomer of the compound of formula I (described below) must be processed over several reaction steps, racemized and recycled.
The object of the present invention is to provide a process which permits the asymmetric hydrogenation of enimines in the form of their salts without the occurrence of the disadvantages described above.